This invention generally relates to optical filter devices.
It has long been observed that cathode ray tube (CRT) displays are relatively unusable in high ambient light environments. This problem has impeded the use of CRTs for navigation and instrument displays in many aircraft, such as jet fighter aircraft, in which high ambient light conditions are normally experienced during day-light operation. This is particularly true for color displays. To overcome this problem, direction sensitive contrast enhancement optical filters have been developed which are mounted on the face-plate of the CRT. These filters effect a greater attenuation of ambient light that is scattered from the phosphor surface of a CRT than light that is generated by the phosphor. Thus a greater difference in brightness levels is achieved between the displayed data and the unactivated phosphor surface on which the data is displayed. As this difference in levels is increased, the display is seen more readily by the observer and is thus more effective. This is of great importance for CRT displays that are intended to be used in high ambient light environments.
These contrast enhancement filters typically comprise a plate having a large number of very small holes therethrough. The plate and the interior areas of all the holes are blackened to absorb ambient light that strikes it. Only the ambient light that goes through the holes, which is a small percentage of the ambient light, strikes the face of the CRT. Such filters allow an observer to see the display on the face of the CRT within a cone of vision in front of the CRT that is a function of the hole size and the thickness of the plate from which the filter is made. In the prior art, use of contrast enhancement filters of the type just described has been limited due to Moire interference patterns they create. This is especially true when used with shadow mask color CRTs.
Another solution to the useable display problem has been to place a spectrally selective filter matched to the spectral output of the CRT in front of the faceplate of a monochromatic CRT. While these filters selectively block light that is not the color of the filter from striking the phosphor, they are effective only when they transmit a small percentage of the light generated by the phosphor of the CRT to the observer. Typical percentages are from 10% to 20%.
Thus, there is a need in the art for a filter device which provides contrast enhancement, does not create Moire interference patterns and which passes a greater amount of useable light than heretofore possible in the art.